Because of their excellent gas barrier properties, transparency and mechanical strength, saturated polyesters such as polyethylene terephthalate are widely used for containers such as bottles. Particularly, the bottles obtained by biaxial stretching blow molding (draw blow molding) of polyethylene terephthalate are excellent in transparency, mechanical strength, heat resistance and gas barrier properties, so that they have been extensively used as containers (PET bottles) to be filled with drinks such as juice, soft drinks and carbonated beverages.
Such bottles are generally produced by a process comprising molding a saturated polyester into a preform having a neck part and a body part, inserting the preform in a mold of given shape, and subjecting the preform to stretching blow molding to stretch the body part, thereby producing a bottle having a neck part and an oriented body part.
The polyester bottles, particularly polyester bottles for drinks such as juice, are required to have heat resistance high enough for heat sterilization of the contents therein, and therefore the bottles are generally further subjected to heat treatment (heat setting) after the blow molding to improve the heat resistance.
In the polyester bottles obtained by the above process, the neck parts are unstretched and inferior to the stretched body parts in the mechanical strength and the heat resistance. In general, therefore, the neck parts of the preforms are heated to crystallize prior to the blow molding, or the necks of the bottles obtained by blow molding are heated to crystallize, thereby improving the neck parts in the mechanical strength and the heat resistance.
In recent years, the sizes of bottles produced from the polyester resins (particularly polyethylene terephthalate) tend to be made smaller. In case of such small-sized bottles, the contact area between the contents and the bottle body part per unit volume is increased, and thus loss of gas or permeation of oxygen from the outside may have a marked influence on the contents, resulting in decrease of shelf life of the contents. Accordingly, the polyester resins are required to have more excellent gas barrier properties than before.
In order to improve the heat resistance and the gas barrier properties of the polyester resins, an attempt to blend polyethylene terephthalate with polyethylene isophthalate has been proposed (see Japanese Patent Publication No. 22302/1989). The blend of polyethylene terephthalate and polyethylene isophthalate, however, generates acetaldehyde when it is melt kneaded at a high temperature to improve compatibility, and this causes problems such as change of taste of the contents filled in the container and lowering of transparency. Further, the polyethylene isophthalate adheres to the screw to prolong the residence time, and this causes another problem of scorching. Furthermore, when polyethylene isophthalate is amorphous, polyethylene terephthalate is required to be dried by an ordinary drier, then cooled and blended in a dry state with the polyethylene isophthalate, followed by molding the blend. Therefore, the cost of equipment for various steps from drying to molding is great, and much space is necessary for the equipment.
To cope with the above problems, there has been proposed a polyester comprising ethylene glycol and a dicarboxylic acid component which comprises terephthalic acid as a major ingredient and isophthalic acid. This polyester, however, does not always have sufficient heat resistance and gas barrier properties and sometimes generates acetaldehyde. Accordingly, development of polyesters having further improved heat resistance and gas barrier properties and hardly generating acetaldehyde is desired.